The invention is applicable particularly but not exclusively to Stirling engines of the multi-cylinder double-acting type. Typical engines of this type have a hot working chamber at one end, normally the upper end and a cold working chamber at the other end of each cylinder separated by the piston, each of these hot and cold working chambers being connected respectively to a cold or hot working chamber of another cylinder. In this way, four closed working volumes are established in each of which the required working fluid is permanently entrapped. Conventional lubricants can not normally be used within the working volume because the lubricant carbonises and carbonised deposits interfere with heat transfer capability.
Differential thermal expansion between a piston and its cylinder in a Stirling engine can be a particularly severe problem. Pistons are normally supported to slide in a bore by axially spaced annular bearing pads which operate in an oil free environment and normally have a high expansion coefficient. The cylinder in the region of the upper bearing pad is normally liquid cooled and so does not experience wide temperature differences or large degrees of expansion. It may also be convenient to make it from a ferrous metal which has a relatively low expansion coefficient. In contrast, the piston may need to operate over a much wider range of temperatures and may be of light alloy with high expansion coefficient. The relatively high expansion of the piston itself and of a bearing pad carried on the piston can be such as to require excessive clearance between the piston and cylinder at low temperatures. The extent of this clearance can be such as to allow substantial angular movement of the piston. A Stirling engine piston often carries a high crown known as a displacer which runs close to but does not contact the cylinder. Lateral movement of the displacer on tilting of the piston, is magnified by its distance from the bearing pads and there can be a risk of inadvertent contact by the displacer with the cylinder wall at low temperatures. Misalignment caused by large clearances can also lead to high wear rates due to edge loading.
If the engine over-heats and clearances diminish to the extent that friction increases, the piston temperature tends to rise more than the cylinder temperature, tending to result in seizure. To guard against this problem during over-heating, a still larger clearance is required at low temperature. Achieving a satisfactory seal in conjunction with low wear and low friction between the piston and cylinder over a complete range of working temperatures and clearances can also be difficult to achieve and has been a major factor in limiting the commercial success of Stirling engines. The problem is even greater with engines requiring a mechanical drive connection to the piston than with free piston engines because a mechanical drive normally involves a side load on the piston.
The arrangement of guiding a piston in a cylinder by annular pads on the piston engaged with the cylinder wall can thus be disadvantageous in a Stirling engine with a mechanical coupling. An object of the present invention is to provide a piston guide arrangement in which these problems are reduced.